arXiv subjects

M. Neubert

Publications and source records attributed to M. Neubert.

At least 37 records · Page 2Linked to original sources

arXiv2001

QCD Factorization in B -> pi K, pi pi Decays and Extraction of Wolfenstein Parameters

In the heavy-quark limit, the hadronic matrix elements entering nonleptonic $B$-meson decays into two light mesons can be calculated from first principles including ``nonfactorizable'' strong-interaction corrections. The $B\toπK,ππ$ decay amplitudes are computed including electroweak penguin contributions, SU(3) violation in the light-cone distribution amplitudes, and an estimate of power corrections from chirally-enhanced terms and annihilation graphs. The results are then used to reduce the theoretical uncertainties in determinations of the weak phases $γ$ and $α$. In that way, new constraints in the $(\barρ,\barη)$ plane are derived. Predictions for the $B\toπK, ππ$ branching ratios and CP asymmetries are also presented. A good global fit to the (in part preliminary) experimental data on the branching fractions is obtained without taking recourse to phenomenological models.

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arXiv2000

QCD factorization for $B\toπK$ decays

We examine some consequences of the QCD factorization approach to non-leptonic B decays into $πK$ and $ππ$ final states, including a set of enhanced power corrections. Among the robust predictions of the approach we find small strong-interaction phases (with one notable exception) and a pattern of CP-averaged branching fractions, which in some cases differ significantly from the current central values reported by the CLEO Collaboration.

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arXiv2000

QCD factorization for exclusive, non-leptonic B meson decays: General arguments and the case of heavy-light final states

We provide a rigorous basis for factorization for a large class of non-leptonic two-body $B$-meson decays in the heavy-quark limit. The factorization formula incorporates elements of the naive factorization approach and the hard-scattering approach, but allows us to compute systematically radiative (``non-factorizable'') corrections to naive factorization for decays such as $B\to Dπ$ and $B\to ππ$. We discuss the factorization formula for a general final state from a general point of view. We then consider factorization for decays into heavy-light final states (such as $B\to Dπ$) in more detail, including a proof of the factorization formula at two-loop order. Explicit results for the leading QCD corrections to factorization are presented and compared to existing measurements of branching fractions and final-state interaction phases.

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arXiv1999

B -> X_u l nu decay distributions to order alpha_s

An analytic result for the O(alpha_s corrections to the triple differential B -> X_u l nu decay rate is presented, to leading order in the heavy-quark expansion. This is relevant for computing partially integrated decay distributions with arbitrary cuts on kinematic variables. Several double and single differential distributions are derived, most of which generalize known results. In particular, an analytic result for the O(alpha_s) corrections to the hadronic invariant mass spectrum is presented. The effects of Fermi motion, which are important for the description of decay spectra close to infrared sensitive regions, are included. The behaviour of perturbation theory in the region of time-like momenta is also investigated

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arXiv1999

QCD Factorization for $B\toππ$ Decays: Strong Phases and CP Violation in the Heavy Quark Limit

We show that, in the heavy quark limit, the hadronic matrix elements that enter $B$ meson decays into two light mesons can be computed from first principles, including `non-factorizable' strong interaction corrections, and expressed in terms of form factors and meson light-cone distribution amplitudes. The conventional factorization result follows in the limit when both power corrections in $1/m_b$ and radiative corrections in $α_s$ are neglected. We compute the order-$α_s$ corrections to the decays $B_d\toπ^+π^-$, $B_d\toπ^0π^0$ and $B^+\toπ^+π^0$ in the heavy quark limit and briefly discuss the phenomenological implications for the branching ratios, strong phases and CP violation.

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arXiv1998

New Bound on gamma from B^+- -> pi K Decays

A bound on the angle gamma of the unitarity triangle is derived using experimental information on the CP-averaged branching ratios for the rare decays B^+- -> pi^+- K^0 and B^+- -> pi^0 K^+-. The theoretical description is cleaner than the Fleischer-Mannel analysis of the decays B^+- -> pi^+- K^0 and B^0 -> pi^-+ K^+- in that the two decay rates differ only in a single isospin amplitude, which has a simple structure in the SU(3) limit. As a consequence, electroweak penguin contributions and strong rescattering effects can be taken into account in a model-independent way. The resulting bound excludes values of cos(gamma) around 0.6 and is thus largely complementary to indirect constraints derived from a global analysis of the unitarity triangle.

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arXiv1998

QCD Anatomy of B -> X_s gamma Decays

We present an updated next-to-leading order analysis of the B -> X_s gamma branching ratio and photon spectrum, including consistently the effects of Fermi motion in the heavy-quark expansion. For the Standard Model, we obtain B(B -> X_s gamma) = (2.57+-0.26^{+0.31}_{-0.36}) * 10^{-4} for the integral over the high-energy part of the photon spectrum with E_gamma^{lab} > 2.2 GeV, where the first error reflects the uncertainty in the input parameters, and the second one the uncertainty in the calculation of Fermi motion. This prediction agrees with the CLEO measurement of the same quantity within one standard deviation. From a reanalysis of the CLEO data, we obtain for the total branching ratio B(B -> X_s gamma) = (2.62+-0.60_{exp}^{+0.37}^{-0.30{th}}) * 10^{-4} using the measured rate above 2.2 GeV, and (2.66+-0.56_{exp}^{+0.43}_{-0.48{th}}) * 10^{-4} using a fit to the photon energy spectrum. Both values are consistent with the Standard Model prediction of (3.29+-0.33) * 10^{-4}. Our analysis contains an improved discussion of renormalization scale dependence and QED corrections. We also discuss the sensitivity of the branching ratio and photon spectrum to hadronic parameters such as the b-quark mass, and to possible contributions from New Physics beyond the Standard Model.

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arXiv1998

Direct CP Violation in B -> X_s gamma Decays as a Signature of New Physics

We argue that the observation of a sizable direct CP asymmetry A_{CP} in the inclusive decays B -> X_s gamma would be a clean signal of New Physics. In the Standard Model, A_{CP} can be calculated reliably and is found to be below 1% in magnitude. In extensions of the Standard Model with new CP-violating couplings, large CP asymmetries are possible without conflicting with the experimental value of the branching ratio for the decays B -> X_s gamma. In particular, large asymmetries arise naturally in models with enhanced chromo-magnetic dipole operators. Some generic examples of such models are explored and their implications for the semileptonic branching ratio and charm yield in B decays discussed.

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arXiv1997

Dispersive Bounds on the Shape of B -> D^(*) l nu Form Factors

Dispersive constraints on the shape of the form factors which describe the exclusive decays B -> D^(*) l nu are derived by fully exploiting spin symmetry in the ground-state doublet of heavy-light mesons. The analysis includes all twenty B^(*) -> D^(*) semileptonic form factors. Heavy-quark symmetry, with both short-distance and 1/m_Q corrections included, is used to provide relations between the form factors near zero recoil. Simple one-parameter functions are derived, which describe the form factors in the semileptonic region with an accuracy of better than 2%. The implications of our results for the determination of |V_cb| are discussed.

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arXiv1997

Two-loop hybrid renormalization of local dimension-4 heavy-light operators

The renormalization of local dimension-4 operators containing a heavy and a light quark field at scales below the heavy-quark mass is discussed, using the formalism of the heavy-quark effective theory. The anomalous dimensions of these operators and their mixing are calculated to two-loop order. Some phenomenological applications are briefly discussed.

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arXiv1997

Higher-order estimates of the chromomagnetic moment of a heavy quark

The leading beta_0^(n-1) alpha_s^n terms in the Wilson coefficient and anomalous dimension of the chromomagnetic operator in the heavy-quark effective Lagrangian are summed to all orders of perturbation theory. The perturbation series for the anomalous dimension is well behaved, while that for the Wilson coefficient exhibits a divergent behaviour already in low orders, caused by a nearby infrared renormalon singularity. The resulting ambiguity is commensurate with terms of order 1/m^2 in the effective Lagrangian, whose corresponding ultraviolet renormalons are identified. An excellent approximation for the scheme-invariant Wilson coefficient at next-to-next-to-leading order in renormalization-group improved perturbation theory is obtained.

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arXiv1997

Two-Loop Anomalous Dimension of the Chromo-Magnetic Moment of a Heavy Quark

We find that the anomalous dimension (in the $\bar{MS}$ scheme) of the chromo-magnetic operator $g_s\bar h_vσ_{μν} G^{μν} h_v$ in the heavy-quark effective theory is γ_{mag} = (C_Aα_s / 2π) [1 + ( 17/18 C_A - 13/18 T_F n_f)α_s/π+ O(α_s^2)] generalizing the one-loop expression known previously. To derive the two-loop result, we use the reparametrization invariance and the virial theorem. After performing infrared subtractions, all two-loop integrals are of propagator type and are evaluated by a recurrence relation for tensor integrals.

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arXiv1996

Renormalization of Velocity-Changing Dimension-Five Operators in the Heavy-Quark Effective Theory

We study the renormalization of operators of the type ${\bar h_{v'}} ΓG^{μν} h_v$ in the heavy-quark effective theory (HQET). We construct the combinations of such operators that are renormalized multiplicatively, and calculate their velocity-dependent anomalous dimensions at the one-loop order. We then show that the virial theorem of the HQET is not renormalized, and that in the limit of equal velocities the anomalous dimension of the chromo-electric operator vanishes to all orders in perturbation theory. This implies an exact relation between renormalization constants, which may help in a future calculation of the two-loop anomalous dimension of the chromo-magnetic operator.

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arXiv1996

Spectator Effects in Inclusive Decays of Beauty Hadrons

We present a model-independent study of spectator effects, which are responsible for the lifetime differences between beauty hadrons. These effects can be parametrized in terms of hadronic matrix elements of four four-quark operators. For $B$ mesons, the coefficients of the non-factorizable operators turn out to be much larger than those of the factorizable ones, limiting considerably the usefulness of the vacuum insertion approximation. Non-factorizable contributions to the lifetime ratio $τ(B^-)/τ(B_d)$ could naturally be of order 10--20%, and not even the sign of these contributions can be predicted at present. In the case of the $Λ_b$ baryon, heavy-quark symmetry is used to reduce the number of independent matrix elements from four to two. In order to explain the large deviation from unity in the experimental result for $τ(Λ_b)/τ(B_d)$, it is necessary that these baryon matrix elements be much larger than those estimated in quark models. We have also reexamined the theoretical predictions for the semileptonic branching ratio of $B$ mesons and charm counting, finding that, given the present theoretical and experimental uncertainties, there is no significant discrepancy with experiment.

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arXiv1995

The Invisible Renormalon

We study the structure of renormalons in the Heavy Quark Effective Theory, by expanding the heavy quark propagator in powers of $1/m_Q$. We demonstrate that the way in which renormalons appear depends on the regularisation scheme used to define the effective theory. In order to investigate the relation between ultraviolet renormalons and power divergences of matrix elements of higher-dimensional operators in the heavy quark expansion, we perform calculations in dimensional regularisation and in three different cut-off regularisation schemes. In the case of the kinetic energy operator, we find that the leading ultraviolet renormalon which corresponds to a quadratic divergence, is absent in all but one (the lattice) regularisation scheme. The nature of this ``invisible renormalon'' remains unclear.

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arXiv1994

Theoretical Update on the Model-Independent Determination of |V_{cb}| Using Heavy Quark Symmetry

In view of new precise measurements of the $\bar B\to D^*\ell \barν$ decay rate near zero recoil, we reconsider the theoretical uncertainties in the extraction of $| V_{cb}|$ using heavy quark symmetry. In particular, we combine our previous estimate of $1/m_Q^2$ corrections to the normalization of the hadronic form factor at zero recoil with sum rules derived by Shifman {\it et al}. to obtain a new prediction with less theoretical uncertainty. We also summarize the status of the calculation of short-distance corrections, and of the slope of the form factor at zero recoil. We find ${\cal F}(1)=η_A \hatξ(1)=0.93\pm 0.03$ and $\hat\varrho^2=0.7\pm 0.2$. Combining this with the most recent experimental results, we obtain the model-independent value $| V_{cb}|=0.040\pm 0.003$.

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arXiv1994

Heavy Quark Masses, Mixing Angles, and Spin-Flavour Symmetry

In a series of three lectures, I review the theory and phenomenology of heavy quark masses and mixing angles and the status of their determination. In addition, I give an introduction to heavy quark symmetry, with the main emphasis on the development of heavy quark effective field theory and its application to obtain a model-independent determination of $|V_{cb}|$.

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arXiv1993

The Subleading Isgur-Wise Form Factor $ξ_3(v\cdot v')$ and its Implications for the Decays $\bar B\to D^{(*)}\ell\,\barν$

We calculate, in the framework of QCD sum rules and to next-to-leading order in perturbation theory, the universal function $ξ_3(v\cdot v')$ which appears at order $1/m_Q$ in the heavy quark expansion of meson weak decay form factors. We find that radiative corrections of order $α_s$ are very important. Over the kinematic range accessible in semileptonic decays, $ξ_3(v\cdot v')$ is proportional to the leading-order Isgur-Wise function $ξ(v\cdot v')$ to very good accuracy. Taking into account all sources of uncertainty, we estimate $ξ_3/ξ=(0.6\pm 0.2)$. This reduces the theoretical uncertainty in the extraction of $|\,V_{cb}|$ from $\bar B\to D\,\ell\, \barν$ transitions. A measurement of the form factor ratio $A_2/A_1$ in $\bar B\to D^*\ell\,\barν$ decays can be used to test our prediction.

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